An irradiated 316 (UNS S31600) stainless steel (SS) alloy consistently showed phosphorus segregation to surfaces at temperatures in which thermal segregation is negligible. Phosphorus segregation did not occur in other irradiated 300 series alloys examined. The segregation model of Lam et al. was used to show that interstitial-solute binding and migration is a likely mechanism for the measured irradiation-induced phosphorus segregation. Surface segregation of sulfur was observed but the segregation mechanism was not established. Segregation of other impurities such as silicon and nitrogen or alloying elements such as chromium were not observed.